(a) Field of the Invention
The present invention relates to an electrolyte for a rechargeable lithium battery and a rechargeable lithium battery including the same, and more particularly, to an electrolyte for a rechargeable lithium battery which can improve low temperature and safety characteristics of the battery.
(b) Description of the Related Art
Rechargeable lithium batteries employ materials into or from where lithium ion can be intercalated or deintercalated for positive and negative active materials.
Transition metal oxide based-compounds are primarily used as the positive active material in the rechargeable lithium battery. Typical examples include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNi1xe2x88x92xCoxO2)(x=0 to 0.5) or lithium manganese oxide (LiiMn2O4)(i=1.0 to 1.5). Carbon-based materials which can reversibly absorb and desorb lithium ions while maintaining structural and electrical properties, in addition to having the same chemical potential as metal lithium when lithium ion is intercalated and deintercalated, are mainly used as the negative active material. Carbon-based materials can be largely classified into two categories of (a) crystalline carbon such as graphite and (b) low crystalline carbon having pseudo-graphite or turbostratic structures. Low crystalline carbon, or amorphous carbon may also be classified into two categories of soft carbon and hard carbon. Soft carbon is produced by heat-treating coal tar or pitch at about 1000xc2x0 C. and hard carbon is produced by carbonizing polymer resin. Crystalline carbon has a high true density, thereby increasing packing efficiency. Crystalline carbon has also an improved voltage flatness and good reversible charge and discharge properties. However, crystalline carbon, has a lower charge capacity than low crystalline carbon. In other words, low crystalline carbon has relatively higher charge capacity than crystalline carbon, but possesses the disadvantage of having extremely high irreversible charge and discharge properties.
The type of electrolytes used is critical to battery performance. The electrolytes include lithium salt such as LiPF6 and organic solvents. The organic solvents used in electrolyte must have various properties including less reactivity with lithium; low internal resistance which helps the movement of lithium ions between the positive and negative electrodes; thermal safety in over a wide range of temperatures; good compatibility with other battery components such as the negative and positive electrodes, and in particular with negative active materials; and a high dielectric constant to increase the amount of lithium dissolved. The organic solvents that satisfy these conditions and therefore are typically used include cyclic carbonate such as propylene carbonate and ethylene carbonate, and chain carbonate such as dimethyl carbonate and diethyl carbonate. Also used as an organic solvent in electrolyte are 1,2-dimethoxyethane, diethoxyethane or a mixture thereof.
Among these materials, propylene carbonate has a good compatibility with amorphous carbon and good low temperature characteristics because it has a low melting point of xe2x88x9249xc2x0 C. In addition, propylene carbonate can dissolve a large amount of lithium salt because it has high dielectric constant. However, propylene carbonate has various disadvantages such as a high viscosity. Furthermore, when crystalline negative active materials such as graphite are used together with propylene carbonate, propylene carbonate dissolves as it is inserted into between negative active material layers, thereby generating propylene gas and lithium carbonate which cause a decrease in capacity and an increase irreversible capacity during charging. Irreversible capacity is primarily caused by structural characteristics of the carbon-based active material and varies according to a reduction of the electrolyte on a boundary where lithium contacts the carbon, and also according to an electrolyte passive layer formed on the surface of carbon.
On the other hand, ethylene carbonate, also listed above as a commonly used organic solvent, does not react with the graphite negative active material and also has a high dielectric constant such that it is applicable to a battery using the crystalline carbon. However, ethylene carbonate has a high viscosity and a high melting point of about 36xc2x0 C. such that low temperature characteristics are not obtained.
Furthermore, chain carbonate (aliphatic carbonate) such as dimethyl carbonate (DMC) and diethyl carbonate (DEC), also stated above as a commonly used organic solvent has a low viscosity and is easily intercalated into the carbon layers, decreasing irreversible capacity. In addition, chain carbonate has a low reactivity with lithium. However, because chain carbonate has a low dielectric constant, it can not dissolve a large amount of lithium salt. With regard to DMC in particular, dimethyl carbonate can be used in high current and high voltage batteries because dimethyl carbonate has high dielectric constant, but this material has a high melting point of 4.6xc2x0 C. such that it has bad low temperature characteristics. Further, regarding organic solvents such as dimethylformamide and acetonitrile although these materials have a high dielectric constant, they have good reactivity with lithium, thereby rendering dimethylformamide and acetonitrile difficult to use as organic solvents.
In order to compensate for the various disadvantages of the organic solvents of the electrolyte solution described above, methods have been disclosed in recent times in which two or more solvents are mixed.
U.S. Pat. No. 5,639,575 discloses an electrolyte including ethylene carbonate/dimethyl carbonate to which diethylene carbonate, having good low temperature characteristics, is added. In this case, when compared with an electrolyte including ethylene carbonate/dimethyl carbonate in which diethylene carbonate is not added, the added version naturally has improved low temperature characteristics, but the difference is not substantial. Furthermore, when this electrolyte is used in a battery, safety problems occur because the electrolyte causes the temperature at which the active material decompose to be low and because the electrolyte causes increases a high quantity of heat when the active material is decomposed.
It is an object of the present invention to provide an electrolyte for a rechargeable lithium battery having good low temperature and safety characteristics.
It is another object of the present invention to provide a rechargeable lithium battery including the electrolyte.
These and other objects may be achieved by an electrolyte for a rechargeable lithium battery including a non-aqueous solvent and a lithium salt. The non-aqueous solvent includes cyclic carbonate, chain carbonate and alkyl acetate.
The present invention further includes a rechargeable lithium battery having a positive electrode with a transition metal oxide-based active material and a negative electrode with a carbon-based active material. A separator is interposed between the negative and positive electrodes. The positive and negative electrodes and the separator are all saturated with an electrolyte. The electrolyte includes cyclic carbonate, chain carbonate and alkyl acetate.